One of the most devastating chemical attacks to take place during the six-year civil war in Syria left dozens of people – including many children – dead or critically injured in the rebel-held province of Idlib on Tuesday.

At least 70 people died in what activists and Western leaders have described as an airstrike conducted by the Syrian military in the northern town of Khan Sheikhoun on Tuesday.

The US launched a salvo of 59 cruise missiles on Shayrat airfield and nearby military infrastructure controlled by Syrian President Bashar Assad, at dawn local time on Friday in response to the attack.

Here’s what happened this week and how the conflict escalated to this point.

Early on Tuesday, a military plane dropped a bomb on a building in the center of rural Khan Sheikhoun. Victims and medical professionals in the area said the large cloud of smoke caused many residents to pass out.

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A man carries the body of a dead child after what rescue workers described as a gas attack in Khan Sheikhoun.

The Syrian Observatory for Human Rights first said at least 58 people, among them 11 children, passed out from inhaling the toxic chemicals released into the air. Many others died later after the initial exposure.

Minutes later, three strikes hit the center of town, exploding in the roads and surrounding buildings.

Doctors from the surrounding areas said over 500 people were rushed to clinics. “Most of the hospitals in Idlib province are now overflowing with wounded people,” Mounzer Khalil, head of Idlib’s health authority, told Reuters.

The chemical, which the World Health Organization said could be a nerve agent, caused many people to choke, foam at the mouth, and writhe in pain before passing out. “It was like a winter fog,” Mariam Abu Khalil, a teenager who saw several residents of her hometown die, told The New York Times.

On Wednesday, Western and Middle Eastern leaders observed a moment of silence at a Brussels conference aimed at finding solutions to end the war. The chemical attack added extra urgency to the discussion.

The deadliest war of the 21st century started when Assad ordered the killing and imprisonment of protesters amid the Arab Spring in 2011. The conflict has escalated to involve Western powers and Russia, and more than 450,000 Syrians have been killed in sieges, military strikes, and air bombings.

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A boy pushes a wheelchair along a damaged street in an east Aleppo neighborhood in January.

International groups have accused the Syrian government of using chemical attacks several times. The deadliest one took place in 2013 when aerial drops of the nerve agent sarin killed over 1,000 people near the country’s capital.

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A Civil Defense member carries a damaged canister from what activists said was a chlorine gas attack in Idlib in May 2015.

A 2017 United Nations report found that both sides committed war crimes in the fight for Aleppo at the end of 2016. Civilians were killed and critically injured in the crossfire.

The civil war has spurred the largest humanitarian conflict since World War II, with more than 5 million Syrians seeking refugee status in other countries. Nearly half of Syria’s citizens have been forced to flee their homes.

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Syrian refugee girls blow soap bubbles next to a fence at the Greek-Macedonian border at a makeshift camp for refugees and migrants in March 2016.

The war has had an especially devastating effect on Syrian children. Many have been killed, recruited as soldiers, or stranded in refugee camps without adequate access to food.

The Trump administration has offered mixed messages on possible ways to resolve the Syrian conflict. Breaking with the Obama administration, the US ambassador to the UN, Nikki Haley, has said that forcing Assad from power is no longer a priority.

President Donald Trump condemned the attack and has previously urged against using military action in Syria. It’s unclear whether the US or Russia will strike back against the perpetrator of the chemical attack.

In the early hours of April 7, Trump launched a salvo of 59 cruise missiles on Shayrat airfield and nearby military infrastructure controlled by Assad in response to the attack.

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U.S. Navy guided-missile destroyer USS Porter (DDG 78) conducts strike operations while in the Mediterranean Sea which U.S. Defense Department said was a part of cruise missile strike against Syria on April 7, 2017.

“It is in this vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons,” said Trump shortly after ordering the attack. This was the first international attack on the Syrian government during six years of civil war.

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President Donald Trump speaks with reporters on Air Force One while in flight from Andrews Air Force Base, Md., to Palm Beach International Airport, Fla., Thursday, April 6, 2017.

Continuing to support Assad’s government, Russia condemned the strikes and said that it would “inflict major damage on US-Russia ties.” The Syrian War has often been called the latest proxy war between the West and Russia.